The present invention relates to an arrangement and a method for converting thermal energy to mechanical energy.
When fuel is burnt in a combustion engine in a vehicle, chemical energy is converted to mechanical energy to propel the vehicle. However, a substantial part of the chemical energy is converted to thermal energy which is released to the surroundings in various ways. An example of this is the thermal energy in the exhaust gases which are discharged to the surroundings. Another example is the thermal energy present in various kinds of warm media in the vehicle which is actively cooled away in various types of cooling devices. An example of such warm media is the coolant in cooling systems which cool the engine and possibly other components of the vehicle. The warm coolant is usually cooled in a radiator by surrounding air at the front portion of the vehicle. Many combustion engines are supercharged, which entails compressed air being supplied to them. The charge air is cooled in at least one charge air cooler before being led into the engine. To reduce discharges of nitrogen oxides, part of the exhaust gases may be recirculated. The recirculating exhaust gases are cooled in at least one EGR cooler before being mixed with the charge air and led into the engine etc.
WHR (waste heat recovery) systems are used to convert the thermal energy to mechanical energy. A WHR system comprises a circuit with a pump which circulates a medium in a line circuit. The line circuit comprises an evaporator in which the medium is vaporised by heat from a heat source, and a turbine which is driven by the vaporised medium. When the medium expands through the turbine, part of the medium's thermal energy is converted to mechanical energy. This mechanical energy may be utilised for various operating purposes or be converted to electrical energy.
WHR systems are so dimensioned as to be of high efficiency at a certain load. Where a heat source with a varying temperature is used, the load is not always within the range in which the WHR system is at its greatest efficiency. Conventional WHR systems are therefore not particularly effective when taking thermal energy from heat sources which have varying temperatures.
JP 08144850 refers to a system for recovery of thermal energy from exhaust gases. The system comprises a line circuit with circulating mixtures of water and ammonia. The system comprises a high-pressure turbine and a low-pressure turbine to extract mechanical energy from the exhaust gases in different operating states in which the exhaust gases are at varying temperatures. This system is complicated and expensive to manufacture.